(1) Field of the Invention
The present invention relates to an oxide superconductor of the Bi--Sr--Ca--Cu--O type having a high critical temperature (Tc) and a high critical current density (Jc).
(2) Description of the Related Art
Metal type superconductors represented by NbTi and Nb.sub.3 Sn are now used as the superconductor. The critical temperature Tc (the temperature at which the material becomes superconductive) of these superconductors is about 20 K. Recently, a composite oxide comprising a transition metal, an alkaline earth element and copper, which shows a superconductive phenomenon above 77 K., is reported in Physical Review Letters, 58 (1978), pages 908-910, and in case of this composite oxide, the superconductive phenomenon can be attained at a temperature of liquid nitrogen, and cheap liquid nitrogen can be used instead of expensive liquid helium. Therefore, the application range of the superconductor tends to expand.
Investigations have been made so as to further elevate the critical temperature Tc in these oxide superconductors, and a superconductor of the Bi--Sr--Ca--Cu--O type having a critical temperature of 110 K. or 80 K., recently proposed in Jap. J. Appli. Letters, 27 (1988), L209, attracts attention.
However, in the case where the above-mentioned superconductor of the Bi--Sr--Ca--Cu--O type is prepared as a sintered body (bulk body), the phase of the critical temperature Tc of 110 K. and the phase of the critical temperature of 80 K. are present in the mingled state in the sintered body, and the 80 K. phase acts as an impurity, and therefore, it is impossible to elevate Tc of the sintered body.
Incidentally, these known 110 K. and 80 K. phases are composed of Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10+.delta. and Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.8+.delta.' respectively.
As regards the technique of forming a sintered body composed solely of the 110 K. phase, it is reported in Takano et al., Jap. J. Appl. Phys., 127, 6 (1988), L1041 to L1043 that the content of the 110 K. phase can be increased by addition of Pb. However, if Pb is added, stable Ca.sub.2 PbO.sub.4 is formed in the grain boundary, and the superconductive characteristics, especially the critical current density (Jc), are lowered.
As the means for overcoming the defect of the system proposed by Takano et al., that is, a low critical current density (Jc), Tanaka et al., propose a process in which a uniaxial pressing step is added during the firing [Jap. Appl. Phys., 27, 9 (1988), L1655 and L1656], and it is taught that the Jc value can be increased according to this process.
However, even if this process is adopted, the Jc value is about 700 A/cm.sup.2 at 0 Oe (Oersted), and with increase of the intensity of the magnetic field, the Jc value is drastically reduced and the Jc value is as small as about 200 A/cm.sup.2 at 100 Oe.